The optical devices are based on a dispersing system with a diffraction grating that is operated near Littrow. In such a system, the following relation is approximately validGmλ=2 sin(β)   (1)
where G is the groove density of the grating, m is the order of diffraction (an integer), λ is the wavelength and β the Littrow angle.
The dispersing system has to be compact in order to keep the size of the optical devices compatible with the requirements for telecommunications equipment as well as for test and measurement equipment.
Wavelength multiplexers, demultiplexers and routers based on a compact dispersing system are available. These devices have been described and progressively developed, in particular in French patents FR-2.479.981, FR-2.496.260, FR-2.519.148, FR-2.543.768, FR-2.579.333, FR-2.731.573, FR-2.743.424, FR-2.761.485, FR-2.763.139, FR-2.764.393, FR-2.765.424, FR-2.765.972, FR-2.779.535, FR-2.803.046 and FR-2.832.882. A first generation of multiplexers-demultiplexers has been marketed under the brand <<STIMAX>>, and subsequently, a second generation has followed under the brand <<MINILAT>>. For a short description of the state of the art, we refer to chapter 3 of the book: Wavelength Filters in Fibre Optics, Herbert Venghaus (Ed.), Springer Verlag, Berlin, 2006.
Wavelength multiplexers and demultiplexers are elementary devices in today's fibre optic long-haul and metro networks. The optical layer of these networks is evolving from static to more dynamic in order to reduce the number of O-E-O (Optical-Electrical-Optical) conversions as well as to remotely optimize transmission capacity for continuously changing traffic demands. This implies that devices are required for monitoring and routing of channels such as, for example, tunable lasers, tunable filters and optical channel monitors. It is noted that these devices also correspond to components and modules incorporated in test and measurement equipment.
FIG. 1A and FIG. 1B represent an optical device 10 of the prior art according to the French patent application FR-2.779.535. The optical device is composed of a fibre array 20 comprising an end face 25 and a compact dispersing system 30. FIG. 1A is a top view of the optical device 10 and FIG. 1B is a side view of the same optical device 10. The optical device 10 can be for example a wavelength multiplexer, demultiplexer, or router. When the optical device 10 is a multiplexer, it comprises N input fibres 21 and a single output fibre 22. When the optical device is a demultiplexer, it comprises a single input fibre 22 and N output fibres 21. When the optical device is a router, it comprises N input fibres 21 and M output fibres 22 or vice versa.
The optical device 10 of FIG. 1A and of FIG. 1B will be described as a demultiplexer. A multiplexer and a demultiplexer are in principle the same device: a demultiplexer is a multiplexer operated in reverse direction. Hereafter, all multiplexers-demultiplexers will be described as demultiplexers. Operation of a multiplexer is obtained from a demultiplexer by reversing beam propagation in the device which implies that input fibre(s) become output fibre(s) and vice versa. It is noted that the optical device 10 is called an optical filter when it has a single input fibre 21 in combination with a single output fibre 22.
The compact dispersing system 30 is composed of a plane mirror 40 with a small aperture 41 in the centre, a concave spherical mirror 60 having a focus and a plane diffraction grating 50 having a dispersion plane. The system 30 ensures wavelength selective conjugation between the end faces of the input fibres 22 and the end faces of the output fibres 21 of the fibre array 20. The optical axis of the system, comprising two parts referenced 31a and 31b, is folded for compactness of the device. The first part of the optical axis 31a runs from the end face 25 of the fibre array 20 through the aperture 41 in the plane mirror 40 up to the centre of the spherical mirror 60, it is perpendicular to the end face 25 of the fibre array 20 as well as to the spherical mirror 60, and it makes an angle a with respect to the plane mirror 40 (FIG. 1A). The diffraction grating 50 makes an angle α with respect to the plane mirror 40 and its dispersion plane coincides with the horizontal plane of the optical device 10. The second part of the optical axis 31b runs from the aperture 41 in the plane mirror 40 to the diffraction grating 50, it makes an angle β with the normal 33 of the grating 50 where β is the Littrow angle and it intersects the first part of the optical axis 31a in the plane of the plane mirror 40, such that the angles α and β are related as follows: α=(β+90°)/2.
FIGS. 1A and 1B show an embodiment of the compact dispersing system 30 using three parts: a wedge prism 42, a plano-concave lens 61, and a substrate 51. The wedge prism 42 with a small aperture in the centre is used to ensure the positioning of the plane mirror 40 with respect to the optical axis 31a, 31b. The tilted face 43 of the wedge prism 42 serves as support for the plane mirror 40 whereas the opposite face 44 of the wedge prism 42 is parallel to the end face 25 of the fibre array 20. The plano-concave lens 61 serves as support for the spherical mirror 60 that resides on its concave face 62. The substrate 51 serves as support for the plane diffraction grating 50 which is formed on the surface of the substrate. The space 32 is filled with air, vacuum or a gas.
The fibre array 20 enables accurate positioning of the end faces of the input fibres 22 and output fibres 21 with respect to the compact dispersing system 30. The fibre end faces are located in the same plane as the end face 25 of the fibre array 20 which is perpendicular to the first part of the optical axis 31a of the dispersing system 30. The fibre end faces are positioned with respect to the aperture 41 in the plane mirror 40 such that outgoing and incoming beams are not affected by the presence of the plane mirror 40.
The fibres 21, 22 are supported by the fibre array 20 which comprises a V-groove block 23 also called V-groove substrate and a V-groove lid 24, between which the fibres 21, 22 are mounted.
FIG. 2A shows a first embodiment of a fibre array 20a for optical devices of the prior art. FIG. 2B shows the end faces of the fibres 21, 22 of the fibre array 20a depicted in FIG. 2A.
The fibre array 20a comprises a V-groove block 23a in which the fibres 21, 22 are placed and a lid 24a covering the fibres 21, 22. The end faces of the M input fibres are referenced by 22a1, . . . , 22am, and the end faces of the N output fibres are referenced by 21a1, . . . , 21an.
The fibre array 20a comprises an end face 25 which is polished such that the end faces of the fibres 21, 22 become part of the end face 25 of the fibre array 20a. In case there is a refractive index difference between the fibres 21, 22 and the adjacent medium, generally, an anti-reflection coating is applied on the end face 25 to eliminate the Fresnel reflection.
The end faces of the fibres 21, 22 are positioned on a straight line as shown in FIG. 2B. In the optical device 10, the straight line is positioned parallel to the dispersion plane of the grating 50. In case of demultiplexing of channels that are equidistantly spaced with respect to wavelength, the end faces of the output fibres 21a1, . . . , 21an are equidistantly spaced at a distance d. The end face of the input fibre 22a1 is separated from the last end face of the output fibre 21an by a minimum distance Δ, typically between 2d and 5d to keep the size of the aperture 41 in the plane mirror 40 limited while minimizing crosstalk effects.
FIG. 3A shows a second, more complex, embodiment of a fibre array 20b for optical devices of the prior art that enables further minimization of crosstalk effects, in particular return loss and directivity which is described in the French patent FR-2.731.573. FIG. 3B shows the end faces of the fibres 21, 22 of the fibre array 20b depicted in FIG. 3A.
The fibre array 20b comprises a first V-groove block 23b and a second V-groove block 24b. The first V-groove block 23b is the substrate in which the output fibres 21 are placed and it serves as the lid for covering the input fibres 22. The second V-groove block 24b is the substrate in which the input fibres 22 are placed and it serves as the lid for covering the output fibres 21. The end faces of the M input fibres are referenced by 22b1,..., 22b, and the end faces of the N output fibres are referenced by 21b1,..., 21bn. It is noted that stacking of two V-groove blocks 23b and 24b requires accurate alignment to ensure parallelism between output fibres 21 of block 23b and input fibres 22 of block 24b. 
Like in the fibre array 20a, the fibre array 20b comprises an end face 25 which is polished such that the end faces of the fibres 21, 22 become part of the end face 25 of the fibre array 20b. In case there is a refractive index difference between the fibres 21, 22 and the adjacent medium, generally, an anti-reflection coating is applied on the end face 25 to eliminate the Fresnel reflection.
The end faces of the fibres 21, 22 are positioned on two parallel straight lines: the end faces of the input fibres 22b1, . . . , 22bm on one line and the end faces of the output fibres 21b1, . . . , 21bn on the other line as shown in FIG. 3B. In the optical device 10, the straight lines are positioned parallel to the dispersion plane of the grating 50. In case of demultiplexing of channels that are equidistantly spaced with respect to wavelength, the end faces of the output fibres 21b1, . . . , 21bn are equidistantly spaced at a distance d. The end face of the input fibre 22b1 is separated from the straight line of the end faces of the output fibres 21b1, . . . , 21bn by a minimum distance D, typically between d and 2d to keep the size of the aperture in the plane mirror 41 limited while minimizing crosstalk effects.
FIG. 4A and FIG. 4B show beam propagation in the optical device 10, where FIG. 4A is a top view of the device 10 and FIG. 4B is a side view of the same device 10.
In case the optical device 10 operates as a demultiplexer, a signal containing a spectral multiplex of channels enters through the input fibre 22, propagates up to its end face 22a1, 22b1 and continues its path by beam propagation in the homogeneous medium 32, where the beam 70 propagates about parallel to the optical axis 31a. The beam 70 passes through the aperture 41 in the plane mirror 40 and diverges until it impinges on the concave spherical mirror 60.
FIG. 5 represents propagation in the single mode optical the fibre 22 up to its end face 22a1, 22b1 followed by beam propagation in the adjacent homogeneous medium 32. Propagation inside the fibre 22 corresponds to a guided mode, having a constant Mode Field Diameter (relative field intensity level of 1/e2), abbreviated as MFD. For example, in the commonly used fibre SMF-28 from Corning, the MFD is around 10.4 μm at a wavelength λ0 of 1550 nm. Beam propagation in the adjacent homogeneous medium 32 starts from the fibre end face 22a1, 22b1 where the beam has its waist equal to the WD. In the adjacent medium 32, the beam 70 diverges according to a cone with an angle θ for the relative field intensity level of 1/e2. The beam waist (MFD) and θ are related as follows:θ=arctan((2λ/(πMFD)).   (2)
For the SMF-28 fibre, a wavelength λ=λ0/n with λ0 (wavelength in vacuum) of 1550 nm, and an adjacent medium 32 with a refractive index n of 1, the angle θ is 5.4°. The cone intersects with the spherical mirror 60 at a propagation distance about equal to the focal length f of the mirror 60. The reflection area of the beam 70 on the mirror 60 has a diameter t of approximately:t≈2f tan(θ)=4f λ/(πMFD).   (3)
For a focal length f of 65 mm in combination with the parameters of the preceding example, the diameter t is about 12.3 mm.
The reflection of the beam 70 on the concave spherical mirror 60 collimates it and reverses its direction of propagation about parallel to the optical axis 31a. Subsequently, it impinges on the plane mirror 40 that reflects it towards the grating 50. A portion of the beam 70 is not reflected due to the small aperture 41 in the plane mirror 40; therefore, increasing the insertion loss and crosstalk effects of the device 10. The beam 70 incident on the grating 50 near Littrow is diffracted back towards the plane mirror 40. The diffraction angularly separates the beam 70, containing a spectral multiplex of channels, into beams as a function of wavelength and therefore separating the channels. Only the beams 71 and 72 corresponding to the first and the last channels are shown in FIG. 4A and FIG. 4B. Subsequently, they impinge on the plane mirror 40 that reflects them towards the concave spherical mirror 60. Again a portion of each beam 71, 72 is not reflected due to the small aperture 41 in the plane mirror 40; therefore, further increasing the insertion loss of the device 10. The reflection of each beam 71, 72 on the concave spherical mirror 60 reverses the direction of propagation and focuses each beam 71, 72 about parallel to the optical axis 31 a through the small aperture 41 in the plane mirror 40 onto the end faces of their corresponding output fibres 21a1, . . . , 21an, 21b1, . . . , 21bn. At these end faces, the size of the beams is about equal to the MFD of the guided mode of the output fibres 21 and propagation continues inside these single mode fibres by their guided mode. This implies that the signal present at the input fibre 22 is demultiplexed at the output fibres 21: each output fibre contains one of the channels of the spectral multiplex, the signal that entered through the input fibre.
FIG. 4B shows the reflection area 45 of all impinging beams 70, 71 and 72 on the plane mirror 40 and FIG. 4B also shows the diffraction area 52 of the impinging beam 70 on the grating 50. These areas, depending on the MFD of the input fibre 22 and the focal length of the dispersing system 30, give an indication of the required size of the different parts. It is noted that the size of the optical device 10 increases when the spectral spacing between the channels decreases because an increase of the focal length of the dispersion system 30 is required. In telecommunications equipment, the height of the optical devices is limited by the distance between the printed circuit boards on which these devices are mounted; for multiplexers and demultiplexers, height of a packaged optical device is typically 14 mm and maximum 16 mm whereas more complex wavelength routing devices can have a height up to 50 mm.
In the French patent application FR-2.779.535, it is indicated that laser diode arrays and photodiode arrays can be used in the optical devices 10, because they have dimensions comparable to those of optical fibres. For example, an optical channel monitor is obtained by replacing the output fibres 21 of a demultiplexer with an array of photodiodes. The implementation is not obvious: a fibre array, similar to FIG. 2 or FIG. 3, needs to be assembled in which the distance Δ or D between end face of the input fibre 22a1, 22b1 and the photodiodes must be kept small. It is feasible when the input fibre 22a1, 22b1 is incorporated into the mount of the photodiode array, but this is more difficult to manufacture.
The optical devices of the prior art described above have a number of drawbacks concerning their insertion loss, their crosstalk effects, their height and their versatility.
The presence of the small aperture 41 in the plane mirror 40 causes an increase in the insertion loss of the optical device 10, because twice a portion of the beam incident on the mirror 40 enters into the aperture 41 instead of being reflected. Moreover, the portion of the beam coming from the spherical mirror 60 enters the aperture 41 about parallel to the optical axis 31a. Therefore, a small part of it couples into the input fibres 22 and output fibres 21 adding to the crosstalk effects.
The aperture 41 in the plane mirror 40 must be kept small to limit the increase of the insertion loss which implies that the distance Δ or D between end faces of the input fibres 22a1, . . . , 22am, 22b1, . . . , 22bm, and the output fibres 21a1, . . . , 21an, . . . , 21b1, . . . , 21bn must also be kept small, FIG. 2B and FIG. 3B. Although, fibre array 20b enables further minimization of crosstalk effects compared to fibre array 20a, some crosstalk effects remain due to the fact that the end faces of the input fibres 22a1, . . . , 22am, 22b1, . . . , 22bm, and the output fibres 21a1, . . . , 21an, 21b1, . . . , 21bn are very close.
For optical devices 10 with a relatively great focal length, the beam diameter at the spherical mirror becomes bigger than the acceptable height for optical devices used in telecommunications equipments. In particular, multiplexers and demultiplexers have a tight limit with respect to height.
As aforementioned, devices are required for monitoring and routing of channels such as, for example, tunable lasers, tunable filters and optical channel monitors. The optical devices 10 can be used for the implementation of these devices, but they are not very well suited from a manufacturing point of view since standard mounts for laser diode arrays and photodiode arrays cannot be directly used. For that reason, the optical devices 10 are not very versatile.